Noob's LED Tester

Introduction: Noob's LED Tester

I find myself, my friends and many other people trying to use mind blowing methods to test an LED.
Many people think that applying 2V or 3V to an LED is easiest way to test it. Perhaps, but not the safest way !

We have, have to limit the current through LED. LED is current controlled device for Electronic's sake !

If you have an insight of Digital Multimeter, you know that it has a Constant Current source when its on LED test function.
This is the best way to test an LED, provide a constant current low enough not to blow it.

Now, sometimes I can't think of a simple solution. I initially designed a "Constant Current Source using LM7805", which can be also made with LM317. And then I halted for a moment, all the 7805 or 317 would do is provide a constant DC voltage. Then its the RESISTOR who limits the current, duh.

So why not to go for a simple way ? Two 1.5V AA batteries (Full Stop).
And that's it, this is not rocket science ! Hence I named it as Noob's LED Tester.

But, seriously, I find it very very handy to test LEDs, even IR LEDs.
Let's begin and have this thing done in less than 10 minutes.

Comments

I think you need to take into account the voltage drop across the LED!

When using a red LED, it drops about 1.5-1.8V. That leaves about 1.5V to drop over your resistor, giving you a 5mA current. That should be just enough to light it.

If you use a white LED with a 3-3.5V fwd voltage you have hardly any voltage drop left for your resistor. At most, with new batteries you might get 0.5V allowing less than 2mA. With anything sort of brand new batteries you are not going to light the LED with this.

Oddly, in order to test all colours of LED you want a high voltage and a big resistor so that the difference between a 1.2V IR LED and a 3.8 V UV LED is comparatively insignificant. You need to watch the power rating, of the resistor if you get too high, however.

If we stick to 1/4 W resistors, and want 10 mA max, we can allow a voltage drop of 0.25/0.01 = 25V. OK, no issue there. So let's use the highest common DC source - 12V DC.

Allowing for the smallest LED forward voltage of say 1V (IR LED) we want or resistor to drop 11V at 10 mA. 11/0.01 = 1100 Ohms. 1.1K, which we can get. In practice 1K would give 11 mA, which is fine too.

Checking the current for a nearly 4V UV LED, we would have 8V left for our resistor, giving a healthy 8mA through a 1K or 7.2 mA through a 1K1.

It's pretty inefficient but it's safe and it's going to light whatever LED you put in its way.